This is confusing. How is this a firefox issue since that isn't part of firefox. You gave very little info how to replicate or screenshot errors for anyone to understand what is going on here.

Hi Francis

If you are making multiple attempts to access that website it may be the case that codes that are coming through are overtaking each other.

Please can you try going through the process just once (one username, password, access code) and see what happens.

Try to enter the code manually instead of pasting from an email.